Process for electroforming low oxygen copper

ABSTRACT

A process for preparing low oxygen copper wherein an electrical current is passed through an aqueous electrolyte containing from 0.001 to 5.0 ounces of a pentose per gallon of aqueous electrolyte and copper having a lower oxygen content than the copper of the anode is deposited at the cathode at a current density of from 20 to 400 amperes per square foot.

United States Patent Jerrold R. Denchfield Canoga Park, Calif.

Jan. 8, 1970 Oct. 26, 1971 North American Rockwell Corporation Continuation of application Ser. No. 650,578, July 3, 1967, now abandoned.

Inventor Appl. No. Filed Patented Assignee PROCESS FOR ELECTROFORMING LOW OXYGEN COPPER 10 Claims, No Drawings U.S. Cl 204/106, 204/3, 204/52 R, 204/52 Y Int. CL C2211 1/16 Field of Search 204/106, 107,108, 52, 52.1

[56] References Cited UNITED STATES PATENTS 657,119 9/1900 Klepetko et al 204/108 694,658 3/1902 Meurant 204/54 1,544,726 7/1925 Coleord 204/52 2,660,555 11/1953 Schloen et a1. 204/108 OTHER REFERENCES A. Kenneth Graham, Electroplating Engineering Handbook, pp. 239- 240, 1962).

William H. Safranek et al., Plating, Vol. 42, pp. 1541- 1546, (1955).

Primary Examiner-G. L. Kaplan Attorneys-Thomas S. MacDonald and L. Lee l-lumphries CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of Ser. No. 650,578, filed July 3, 1967 and now abandoned.

BACKGROUND OF INVENTION Coppers unique properties of high thermal conductivity, high electrical conductivity, ductility, and relatively low cost combine to make it one of the most useful of all metals. It is especially useful in the areas of electrical and thermal transfer. Unfortunately, copper also has the additional characteristic of having its advantageous physical properties reduced spectacularly by the inclusion of relatively small amounts of impurities in the copper. One of the most common impurities, and yet one of the most deleterious, is oxygen. Oxygen in copper affects coppers inherent high ductility, high electrical and thermal conductivity, immunity to deterioration when heating under reducing conditions, high impact strength, strong adherence of oxide scale, good creep resistance, ready weldability and low volatility under high vacuum.

A high grade of copper known as electrolytic tough pitch copper has an oxygen content of only about 0.03 percent to 0.06 percent. However, it should be noted that this is equivalent to 0.27 percent cuprous oxide and that in turn to 7.7 percent copper-cuprous oxide eutectic. This tough-pitch copper is by far the predominating form in which copper is marketed. Although for many purposes this type of copper is entirely satisfactory, in some applications copper with a lower oxygen content would be better suited.

One application in which oxygen is especially deleterious to the copper is in the field of high-temperature hydrogen atmosphere brazing of large parts. It has been found that if the brazing is of sufficient temperature and duration, the hydrogen can combine with the oxygen dissolved in the copper, producing water which manifests itself as voids at the grain boundaries of the copper or as blistering on the surface of the copper. Voids in the grain boundaries tend to make the copper brittle and hence unsuitable for some purposes. Blisters on the surface of the copper make the copper rough and hence unsuitable for some purposes.

Prior Art Attempts have been made in the prior art to reduce the oxygen level of copper to below that found in tough-pitch copper. One prior art method of reducing the oxygen content of tough-pitch copper is the use of a remelting step under a controlled reducing atmosphere, producing a low-oxygen copper. This process has the disadvantage of being difficult to control. Frequently, the low-oxygen copper produced thereby will have an oxygen content well above l parts per million. A second method involves deoxidizing molten electrically refined tough-pitch copper by the addition of a reducing material such as phosphorous, boron or lithium, producing the oxides of the metals and a low-oxygen copper. This process has the disadvantage of tending to leave dissolved reducing metal in the copper, somewhat deleteriously affecting the coppers properties. A third process involves the electroforming of low-oxygen copper from a mineral acid bath containing a wood such as Alleghany White Oak. This process has the disadvantage of being operable at only low-current densities, typically about amps per square foot. At current densities much above this, the current scorches the wood, producing inferior copper at the cathode. Unfortunately, these low-current densities render this process economically unfeasible in many applications.

It is an object of this invention to provide a process for the manufacture of low-oxygen copper and direct manufacture of copper structures.

It is a further object of this invention to provide a process for the manufacture of a copper that is especially suited to brazing in a hydrogen atmosphere.

Further and more specific advantages of this invention will become apparent from the following description.

DESCRIPTION OF INVENTION The process of the instant invention comprises formulating an electrolytic copper plating bath; said bath containing from 5.0 to 0.001 ounces per gallon of a pentose; placing copper to be thereby electroformed and degassed at the anodic position of the bath; energizing the bath with electricity at a current density of from 20 to 400 amperes per square foot; thereby forming copper at the cathodic pole of the bath.

If copper is used as the anode in a plating bath, it will plate upon the cathode. if the anode is relatively large compared to the cathodic structure, this bath can be used to refine copper. It is this step, for instance, which produces electrically refined tough-pitch copper from simple fire refined tough-pitch copper. A typical commercial copper plating cell comprises a metal vessel lined with, for example, polyvinyl chloride, containing a copper electrolyte at a temperature of about F. When using acid sulfate baths, copper can be plated at rates up to about amps per square foot. When using fluoroborate baths, copper can be plated at rates up to about 400 amps per square foot. However, this plating does not have the capacity to lower the oxygen content of the copper to a point where it could be termed oxygen free" copper.

The use of mineral acid baths to plate copper is quite old in the art. Numerous types of acid copper plating formulations have been suggested. These include sulfate-oxalate-boric acid, sulfate, sulfate-oxalate, cuprous chloride-sodium thiosulfate, alkane sulfonate, fluoroborate baths are the most extensively used, since they are simple to formulate and easy to operate and control. Among these two, the fluoroborate is frequently preferred due to its capacity to take high current densities, as high at 400 amps per square foot. The sulfate bath, on the other hand, rarely is operated at current densities above about amperes per square foot. Typically, such acid baths comprise a copper salt such as copper sulfate, or copper fluoroborate and a mineral acid such as sulfuric acid or fluoroboric acid. These baths can be run at temperatures from ambient to about F. The preferred temperature is close to 100 F.

Basic copper plating baths are also quite old in the art, having been in standard use since about 1938. The most common basic bath is a cyanide bath containing such salts as copper cyanide, sodium cyanide and, of course, a base, such as caustic potash. Since copper is plated from the cuprous or monovalent state, twice the thickness of copper will be deposited from the basic bath as from the acid bath for any given current density. For reasons of toxity, pyrophosphate baths are sometimes preferred to cyanide baths. Such a bath is typically comprised of copper pyrophosphate, potassium pyrophosphate and ammonia.

lt has been unexpectedly found that the addition of pentose to these baths result in the electrodeposition of a very highgrade low-oxygen copper and further that this copper may be electrodeposited at rates heretofore unobtainable. All of the pentoses are suitable for use in this invention, i.e., xylose, arabinose, ribose and lyxose. The sugars are used in very small concentrations. A typical solution uses about U 10,000 as much pentose as copper salt by weight. In general, concentrations of from 5.0 to 0.001 oz./gal. of pentose are satisfactory. lf much above 5.0 ozjgal. are used, the application of current will tend to decompose the sugar, contaminating the copper being plated. For reasons of economy of bath formation, concentrations below 5.0 ozJgal. are desirable in many applications. Indeed, an entire order of magnitude less is still workable. Therefore, from 0.5 to 0.00l ozJgal. is a preferred range.

It has been observed that very little of the sugar is required when refining a commercially available low-oxygen copper to acceptable levels. For refining these relatively pure copper sources, concentrations of pentose of only from 0.0l to 0.00] have been found to be suitable.

The copper can be plated, and hence refined, onto a supportive cathode of, for instance, stainless steel, and then melted off of the cathode under an inert atmosphere and cast into bars so as to provide ingots of low-oxygen copper. In some applications, it has been found more economical to electroform copper objects of the desired shape. In this embodiment of the invention, a cathode of the desired configuration, but smaller in every dimension, would be plated with a layer of low-oxygen copper and then machined to precise dimensions.

The addition of specified agents to copper electroplating baths for sundry purposes is old in the art. For instance, some agents are known to produce an increased brightness of the copper plated and others will produce a more tenacious plate. U. S. Pat. No. 694,658, issued to Jules Meurant on Mar. 4, 1902, for instance, describes the improvement of copper plating by the addition of organic compounds, e.g., gum arabic, various sugars. The inventor there recites the use of approximately half as much sugar as copper salt and states that his bath can be used at current densities of between 4.6 amps per square foot and 9.3 amps per square foot. it can be seen that a superficial similarity exists between this issued patent and the instant invention. However, the instant invention, as was previously described, uses very small amounts of sugar in relation to copper salt. Consequently, sugar is present in very small concentration in the bath of the instant invention. The bath can therefore be run at heretofore unrealizably high current densities. If the sugar were at a higher concentration, for instance those of the Meurant patent, it would not stand the current densities of the instant invention, and would break down, contaminating the bath. Further, it is just these high current densities which render this process superior to present processes. If these current densities were to be used in the White Oak process previously referred to, for instance, the wood would be charred, producing inferior copper.

The following examples illustrate the novel process of this invention:

EXAMPLE I Using a copper sulfate solution containing 33 ounces per gallon of copper sulfate, 9 ounces per gallon of sulfuric acid and 0.005 ounces per gallon of xylose, low-oxygen (6 p.p.m.) copper was plated from a copper anode containing at least p.p.m. of oxygen at 60 amperes per square foot or a rate of 0.002 to 0.003 inches per hour.

EXAMPLE ii A bath of the same constituents and concentrations, omitting the xylose, of example I is formulated and run for the same period, at the same current densities and using; the same anodic copper. The copper plate at the cathode is black, indicating the presence of large amounts of copper oxide, and hence oxygen.

EXAMPLE III A copper plating bath is formulated of 60 ounces per gallon copper fluoroborate and 0.0! ounces per gallon of ribose. An oxygen-containing copper is used as the anode. Low-oxygen copper is plated at 350 amperes per square foot, forming degassed copper, superior to the anodic copper, at the cathodic pole of the bath.

EXAMPLE IV A bath of the same constituents and concentrations, omitting the ribose, of example I is formulated and run for the same period, at the same current densities and using the same anode copper. The copper plate at the cathode is black, indicating the presence of large amounts of copper oxide, and hence oxygen. The copper plate at the cathode does not contain less oxygen than the startixefianodic copper.

EXA LE V A copper plating bath is made of 3 ounces per gallon of copper cyanide, 5 ounces per gallon sodium cyanide and 0.33 ounces per gallon of sodium hydroxide. Three ounces per gallon of arabinose is added to the solution and copper is plated at a current density of about 30 amperes per square foot. The copper that is thereby plated contains less oxygen than the anodic copper.

EXAMPLE VI A bath of the same constituents and concentrations, omitting the arabinose, of example I is formulated and run for the same period, at the same current densities and using the same anode copper. The copper plate at the cathode is black, indicating the presence of large amounts of copper oxide, and hence oxygen.

EXAMPLE VI] A copper plating bath is made of 4 ounces per gallon of copper pyrophosphate, 30 ounces per gallon of potassium pyrophosphate, and 0.025 ounces per gallon of ammonia. 0.4 ounces per gallon of lyxose is added to the solution and copper is plated at a current density of about amperes per hour. The copper plated is superior to the anodic copper.

EXAMPLE Vlll A bath of the same constituents and concentrations, omitting the lyxose, of example I is formulated and run for the same period, at the same current densities and using the same anode copper. The copper plate at the cathode is black, indicating the presence of large amounts of copper oxide, and hence oxygen.

lclaim:

l. A process for electrodepositing low-oxygen copper comprising passing an electrical current from a copper anode containing dissolved oxygen through a bath consisting of water, a copper salt electrolyte, mineral acid and from 0.001 to 5.0 ounces of a pentose per gallon of said bath to a cathodic member and plating copper having a lower dissolved oxygen content than the copper of the anode at a current density of from 20 to 400 amperes per square foot on the cathodic member.

2. The process of claim 1 in which the copper electrolyte is copper sulfate.

3. The process of claim 1 in which the copper electrolyte is copper fluoroborate.

4. The process of claim 1 in which the mineral acid is sulfuric acid.

5. The process of claim 1 in which the mineral acid is fluoroboric acid.

6. The process of claim I in which the bath contains from 0.001 to 0.5 ounces of a pentose per gallon of said bath.

7. The process of claim 1 in which the bath contains from 0.001 to 0.01 ounces of a pentose per gallon of said bath.

8. The process of claim 1 in which the current density is from 20 to I25 amperes per square foot.

9. The process of claim 1 in which the bath is heated to a temperature between ambient and F.

10. The process of claim 1 in which the copper electrolyte is copper sulfate, the mineral acid is sulfuric acid and the pentose is xylose.

l i U 

2. The process of claim 1 in which the copper electrolyte is copper sulfate.
 3. The process of claim 1 in which the copper electrolyte is copper fluoroborate.
 4. The process of claim 1 in which the mineral acid is sulfuric acid.
 5. The process of claim 1 in which the mineral acid is fluoroboric acid.
 6. The process of claim 1 in which the bath contains from 0.001 to 0.5 ounces of a pentose per gallon of said bath.
 7. The process of claim 1 in which the bath contains from 0.001 to 0.01 ounces of a pentose per gallon of said bath.
 8. The process of claim 1 in which the current density is from 20 to 125 amperes per square foot.
 9. The process of claim 1 in which the bath is heated to a temperature between ambient and 150* F.
 10. The process of claim 1 in which the copper electrolyte is copper sulfate, the minEral acid is sulfuric acid and the pentose is xylose. 